1. Field of the Invention
The present invention relates generally to fluxbucking transformers and more particularly to fluxbucking line transformers particularly suitable for application in telephone line circuits.
2. Description of the Prior Art
In present day telephone subscriber systems it is required to couple large DC biases on the order of 50 volts thru the primary windings of line transformers to the telephone tip and ring lines of the subscriber sets to provide various DC voltages, such as the voltage required for operation of the telephone carbon microphone, upon which DC voltage the speech signals are modulated, and other signals utilized in telephone subscriber sets. Prior art line transformers which could withstand the required large DC biases without saturation of the transformer core and consequent distortion of the relatively small ac speech modulation generated by the carbon microphone which must be coupled from the line transformer primary windings to the transformer secondary windings to which the 2-wire to 4-wire conversion circuitry is connected require either large cores to avoid saturation by the DC current or complex compensation circuitry. Thus, such prior art transformers as a whole must be very large in relation to a transformer which, while required to couple ac signals is not required to pass a large DC current. In order to reduce this requisite large transformer core size and associated cost, prior art fluxbucking techniques compensate for the DC bias in the transformer by sensing the DC components in the magnetic field with Hall effect sensors or by sensing the DC current through the transformer windings with current sensors to derive control signals for use in generating a fluxbucking current of opposite polarity which is then fed back to the transformer windings to cancel out the DC field. Such prior art control circuitry is both complex, expensive and does not effectively compensate for different DC currents in diffrent transformer windings, such as occur in the primary windings associated with telephone tip and ring lines.